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Reactions of Inorganic High Polymers as a Route to Tailored Solids

12. PERSONAL AUTHOR(S) Harry R. Allcock

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FIU SUB- - Solids, polymers, phosphazenes, synthesis "ke).

ABSTRACT (Continue on reverse if necessary and identify by block number)new class of reactive inorganic polymers--the polyphospiazenes-- can be used as

macromolecular intermiiiates for the synthesis of a wide range of inorganic-organicsolids. By reactions that change the polymer side groups, it is possible to bias theproperties toward elastomers or microcrystalline polymers, liquid crystalline materials,bioerodable solids, solids with bioactive surfaces, solid electrolytes, semicoductors,or ultrastructures. U I-T'

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Technical Report No. 48

REACTIONS OF INORGANIC HIGH POLYMERS AS AROUTE TO TAILORED SOLIDS

by

Harry R. Allcock

Accepted for Publicationin

Solid State lonics

Department of ChemistryThe Pennsylvania State University

University Park, Pennsylvania 16802

Reproduction in whole, or in part, is permitted for any purpose of theUnited States Government.

This document has been approved for public release and sale; itsdistribution is unlimited.

Reactions of Inorganic High Polymers as a Route to Tailored Solids

Harry R. Ailcock

Department of ChemistryThe Pennsylvania State University

University Park, Pennsylvania 16802

Abstract

A new class of reactive inorganic polymers--the polyphosphazenes--

can be used as macromolecular intermediates for the synthesis of a wide

range of inorganic-organic solids. By reactions that change the polymer

side groups, it is possible to bias the properties toward elastomers or

microcrystalline polymers, liquid crystalline materials, bioerodable

solids, solids with bioactive surfaces, solid electrolytes, semi-

conductors, or ultrastructures.

1. Purpose and Rationale

Linear macromolecules provide both an intellectual and a synthetic

starting point for the development of new solid state systems. First, a

study of the interactions between independent macromolecules in the

solid state allows an understanding of the way in which van der Waals,

polar, or ionic forces affect the physical properties of the bulk

material. Second, the joining of macromolecules through covalent

crosslinks provides a method for a progressive change of solid state

properties from those of loosely associated molecules toward those of

amorphous ultrastructure ceramics. And third, for both uncrosslinked

and crosslinked polymers, changes in the composition and geometry of the

2

backbone and side groups bring about profound changes in the solid state

properties.

Most of the known polymer-based solids are derived from organic

polymers, synthesized from petrochemical monomers. Although some

similarities can be detected between organic polymeric solids and

totally inorganic materials, such as ceramics or metalloid semi-

conductors, in general a broad gap in properties exists between the

totally organic and the completely inorganic solid systems. The thermo-

oxidative instability of most carbon-based solids provides one example

of the differences.

An objective of our work has been the synthesis of new macro-

molecules that possess a backbone of inorganic elements, flanked by side

groups that have either organic or inorganic character. Such polymers

occupy a zone of structures and properties between those of classical

organic polymers and those of inorganic solids. The method of

inorganic-organic polymer synthesis employed in this work allows the

side groups to be varied over a wide range of structures. By changing

the side groups, the solid state properties can be biased either toward

those typical of organic polymers (elastomers, microcrystalline

polymers, liquid crystalline solids) or toward those of totally

inorganic solids, such as amorphous ceramics, solid electrolytes, semi-

conductors, or metals.

2. Synthetic Method

The polymers discussed here are called polyphosphazenes. They have

the general molecular structure shown in 1, where n, the degree of

3

N=PIR

n

polymerization, can be as high as 15,000-20,000, and the side groups, R,

can comprise a wide variety of different organic, organometallic, or

inorganic units. Depending on the experimental conditions, the

inorganic backbone may be essentially linear, or highly branched.

The synthetic pathway starts from phosphate rock and atmospheric

nitrogen. Phosphate rock is converted first to elemental phosphorus and

thence to phosphorus pentachloride. Nitrogen is converted to ammonia

and subsequently to ammonium chloride. Phosphorus pentachloride and

ammonium chloride are then allowed to react in a chlorinated organic

solvent to yield the two products shown as 2 and 3. Both materials are

available commercially.

Cl Cl Cl ClN\ / I I

P Cl - P N P - Cl

N NCl Cl N NN I II/, .........

CliPN/PC Cl-P-N-P-Cl 'A -NCl I I I

Cl Cl

3

*Z/; Avoalibility Codes

~ ) Avail and/orDist I bpoL;alDIv'I I

4

II I

,--J

0

0 acN

dad

I Id I I

96 IU

\/ \

0

0

oo 09o

J O 0

C4)

r-4

C.) )

.0z

Uz

U)i

5

Scheme II

PCl Cl R

C 1 N NC1 heat II I

Cl N C1 - U lR n

Replacement of chlorine by

organic groups via nucleophilic

substitution

6

Initially, conditions were found that allowed 2 or 3 to be

polymerized thermally to a linear, uncrosslinked, high polymer (4).2- 4

Polymer 4, a rubbery elastomer in the solid state, is highly reactive

toward reagents that will cleave phosphorus-chlorine bonds. Thus, 4

functions as a macromolecular reaction intermediate for the replacement

of all the chlorine atoms along each chain by organic, organometallic,

or inorganic side groups. In nearly every case, the replacement of

chlorine by an organic unit yields a polymer that is very stable in the

atmosphere and often stable at elevated temperatures.

The physical and chemical properties depend on the side groups

introduced.2- 10 In addition, two or more different types of side groups

can be incorporated, and this further widens the property variations

that can be generated. The overall synthesis pathway is summarized in

Scheme I. Further variations are possible by the polymerization of

organo-substituted cyclic trimers or tetramers, and this option is

illustrated in Scheme II.

Schemes I and 1I

The main advantage of this synthesis route for the preparation of

new solids is as follows. Because the side group structure determines

the solid state character and, because the side groups can be changed so

readily by logical variations in chemical reagents used in the

synthesis, both subtle and gross changes in properties can be generated.

This provides an excellent tool for the study of structure-property

relationships and for the design of improved materials. The following

examples illustrate the manner in which different side groups affect the

7

bulk properties of polyphosphazene solids, and how the reactivity of the

surfaces can be used to modify the biological activity of the material.

3. Effect of Different Side Groups on Bulk Properties and Materials

Chemistry

3.1 Elastomeric solids and microcrystalline materials and glasses.

The linear polyphosphazene backbone has an inherent flexibility that

results from a low barrier to torsion of the -P-N-backbone bonds.

Provided the side groups attached to the phosphorus are small in size or

are themselves flexible, these molecular characteristics give rise to

elastomeric solid state properties. 1 1 Random mixed substitution that

destroys molecular symmetry also favors elasticity by preventing the

growth of microcrystalline domains.6 ,7 9,ll12 Polyphosphazene elastomers

that bear alkyl ether side groups function as solid coordinative

"solvents" for salts such as lithium triflate.13 Such systems are

currently under investigation for use in lightweight rechargeable

batteries.

However, increases in the size and rigidity of the side groups

retards macromolecular reorientation and favors microcrystallization.

Polyphosphazenes such as these (for example, those with OCH2CF3 or OC6 H5

side groups) superficially resemble organic microcrystalline polymers,

such as polyethylene, in appearance, but are more resistant to

photolysis or oxidation.1 ,2 Flat, "stackable" side groups, such as

those bearing metal phthalocyanines,13 impose additional ordered

character which, in some cases, can be utilized to generate weak

electrical semi-conductivity. Aromatic azo side groups linked to the

chain via flexible spacer groups yield liquid crystalline

materials.15,16

8

Species that combine some of the properties of polymers and metals

have also been synthesized. Examples include polyphosphazenes with

transition metals bonded directly to the backbone, 17 and others with

ferrocene and/or ruthenocene units that function as side groups.18

Species such as these are of interest as polymeric electrode mediator

solids.19

3.2 Surface reactivity. The surface character of a solid is an

important factor that determines its suitability for many technological

and biomedical applications. Use of the macromolecular substitution

route (Scheme I) provides the primary means for modification of surface

hydrophilicity or hydrophobicity. For example, fluoroalkoxy side groups

generate hydrophobic surface character, whereas, methylamino,20

glucosyl,2 1 or glycery12 2 side groups generate hydrophilicity.

However, a secondary method of surface-tailoring involves the

reactions of side groups that lie at the surface of the solid. For

example, the aryloxyphosphazene polymer [NP(OC6H5 )2 ]n undergoes surface

nitration. The arylnitro units formed then serve as sites for further

surface chemistry. For example, subsequent conversion of the nitro

groups to amino units has provided sites for the covalent binding of

enzymes2 3 or dopamine24 to the surface, with retention of the biological

activity.

3.3 Crosslinking as a route to membranes or ceramics. Lightly

crosslinked polymers possess many of the characteristics of uncross-

linked macromolecules. The main difference is that the crosslinks

prevent dissolution of the polymer in suitable solvents, although the

9

solid may absorb appreciable amounts of solvents and swell to a gel.

Light crosslinking of a water-soluble polyphosphazene will yield a

polymer that swells in water to form a hydrogel.25

On the other hand, extensive crosslinking yields non-swellable

ultrastructures. Poly(aminophosphazenes) of formula [NP(NHR)2 ]n react

at moderate temperatures (200-500*C) by side group elimination

reactions with with concurrent formation of P-N-P crosslinks.26 Some of

the resultant ceramics have compositions approaching that of phosphorus

nitride. Other polyphosphazenes with carborane side groups have been

pyrolyzed to ceramic coatings containing phosphorus, nitrogen, boron,

and carbon.275

Acknowledgments

This work was supported by the U.S. Army Research Office, the Air

Force Office of Scientific Research, Office of Naval Research, and the

N.I.H. through the National Heart, Lung, and Blood Institute.

References

E1l For a review of the synthetic background see, H. R. Allcock,

Phosphorus-Nitrogen Compounds (Academic Press, New York, 1972).

[21 H. R. Allcock, R. L. Kugel, J. Am. Chem. Soc. 87 (1965) 4216.

(31 H. R. Allcock, R. L. Kugel, Inorg. Chem. 5 (1966) 1016.

(41 H. R. Allcock, R. L. Kugel, K. J. Valan, Inorg. Chem. 5 (1966)

1709.

(51 H. R. Allcock, Chem. Eng. News 63 (1985) 22.

[6] R. E. Singler, N. S. Schneider, G. L. Hagnauer, Polym. Eng. and

Sci. 15 (1975) 321.

10

[7] D. P. Tate J. Polym. Sci., Polym. Symp. 48 (1974) 33.

[8] H. R. Allcock in: Inorganic and Organometallic Polymers, eds.

M. Zeldin, K. J. Wynne, H. R. Allcock, ACS Symp. Ser. 360 (1988)

250.

[91 R. E. Singler, M. S. Sennett, R. A. Willingham in: Inorganic and

Organometallic Polymers, eds. M. Zeldin, K. J. Wynne, H. R.

Allcock, ACS Symp. Ser. 360 (1988) 268.

(101 H. R. Penton in: Inorganic and Organometallic Polymers, eds.

M. Zeldin, K. J. Wynne, H. R. Allcock, ACS Symp. Ser. 360 (1988)

277.

[111 H. R. Allcock, M. S. Connolly, J. T. Sisko, A. Al-Shali, Macro-

molecules 21 (1988) 323.

[121 S. H. Rose, J. Polym. Sci. B 6 (1968) 837.

[131 P. M. Blonsky, D. F. Shriver, P. E. Austin, H. R. Allcock,

Polym. Mater. Sci. Eng. 53 (1985) 118.

(141 H. R. Allcock, T. X. Neenan, Macromolecules 19 (1986) 1495.

[151 C. Kim, H. R. Allcock, Macromolecules 20 (1987) 1726.

[161 R. E. Singler, R. A. Willingham, R. W. Lenz, A. Furakawa,

H. Finkelmann, Macromolecules 20 (1987) 1728.

[17] H. R. Allcock, M. N. Mang, G. S. McDonnell, M. Parvez, Macro-

molecules 20 (1987) 2060.

(181 H. R. Allcock, G. H. Riding, K. D. Lavin, Macromolecules 20 (1987)

6.

(191 R. A. Saraceno. G. H. Riding, H. R. Allcock, A. G. Ewing, J. Am.

Chem. Soc. 110 (1988) 980.

11

[201 H. R. Allcock, M. Gebura, S. Kwon, T. K. Neenan, Pinmaterials

(in press).

(211 H. R. Allcock, A. G. Scopelianos, Macromolecules 16 (1983) 715.

[22] H. R. Allcock, S. Kwon, Macromolecules (submitted).

[231 H. R. Allcock, S. Kwon, Macromolecules 19 (1986) 1502.

[241 H. R. Allcock, W. C. Hymer, P. E. Austin, Mcromolecules 16 (1983)

1401.

[251 H. R. Allcock, S. Kwon, G. H. Riding, R. J. Fitzpatrick, J. L.

Bennett, Biomaterials (in press).

(261 G. S. McDonnell (unpublished work)

[271 L. L. Fewell, R. J. Basi, J. Appl. Polym. Sci. 28 (1983) 2659.

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